Proliferation of storage traffic due to disaster recovery and business continuity requirements, combined with the high cost associated with downtime or loss of data, justifies the need for storage area network (SAN) extensions over geographically dispersed areas. Enterprises as well as service providers are seeking affordable solutions to interconnect their SAN islands and implement geographically dispersed data recovery solutions. SAN extension solutions help to interconnect customer storage islands which are geographically dispersed and are the key enabler for disaster recovery and business continuance applications.
One of the most popular protocols for SANs is Fibre Channel. All information in Fibre Channel is transmitted as transmission words which are groups of four transmission characters, each of which is one byte in length. Some transmission words have a K28.5 character as the first transmission character. Groups of four transmission characters which have a K28.5 character as the first transmission character are called ordered sets. Some ordered sets mark the beginning and end of frames (frame delimiters). Others convey information in between frames in the form of primitive indications (a single ordered set) and primitive sequences (a stream of the same ordered set). Examples of ordered sets are: start of frame (SOF), end of frame (EOF), idle, receiver_ready (R_RDY), and several others. Examples of a stream of the same ordered set are link reset (LR), link reset response (LRR), not operational (NOS), offline (OLS), and others. Some of the primitive sequences are used to set up the communication channel that is needed before data transfer can occur between two Fibre Channel enabled devices.
The three primary technologies available to extend the Fibre Channel protocol across a wide-area network (WAN) are SAN over dense wavelength-division multiplexing (DWDM), IP, and SONET/SDH. The International Telecommunications Union (ITU) has developed a transparent-mode version of the generic framing procedure (GFP-T) that allows designers to package SAN traffic into a virtually concatenated pipe to use fully the available bandwidth of the Sonet/SDH network to fully leverage their capabilities.
The GFP-T utilizes a 64B/65B encoding technique to map 8B/10B data and control codes into eight payload bytes and a flag bit. The leading flag bit indicates whether control codes are included in the 64B/65B code where a flag value of 1 indicates the presence of control codes. A control code byte consists of three fields, the first being a single bit set to logic zero if the byte holds the last control code in the 64B/65B code. The second field is a 3-bit address indicating the original location of the control code in a client data field. The third field is a 4-bit field encoding the identity of the control code.